Rose Black spot

Chemical control, Disease, Organic control, Pest, Roses -

Rose Black spot

Rose black spot:

Rose black spot is a fungal disease of roses where purple or black spots develop on leaves, which often drop early.

Quick facts
Common name: Rose black spot
Scientific name: Diplocarpon rosae
Plants affected: Roses
Main symptoms: Purple or black spots on leaves, leaves falling early
Caused by: Fungus
Timing: Spring onwards

These are variable, depending on the rose variety and the strain of the fungus.

You may see the following symptoms:

Typically, a rapidly enlarging purplish or black patch appears on the upper leaf surface, with diffuse and radiating strands of the fungus sometimes just visible.
Leaf tissues may turn yellow around the spots and the leaf often drops, even though other parts are as yet unaffected
At other times, the yellow colour does not appear, but infected leaves still drop
Sometimes, the spots remain relatively small and the leaf does not drop
Small, black, scabby lesions may also appear on young stems
Badly affected plants can shed almost all their leaves and their vigour is greatly reduced. The symptoms are so severe that, anecdotally, the disease has been blamed for a decline in the popularity of roses in UK gardens in recent decades.

Non-chemical control
Collect and destroy fallen leaves in the autumn, or bury under a layer of mulch. Prune out all stem lesions in spring before leaves appear. These actions will help delay the onset of the disease, but are of limited value because spores are bound to blow in on wind-blown rain from elsewhere.

Popular garden varieties of hybrid teas, floribundas, climbers and patio types are usually susceptible. Gardeners may gain a few years' respite by planting the newest varieties which claim resistance, but as discussed above, this usually does not last. Older species types are little affected.

Chemical control
Fungicides containing difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Bayer Garden Systhane Fungus Fighter, several formulations), tebuconazole (Bayer Garden Multirose Concentrate 2), triticonazole (Scotts Fungus Clear Ultra) and Plant and Fish Oil Blends (Vitax Organic 2 in 1) may all be used. Some formulations of myclobutanil, tebuconazole and triticonazole contain insecticides to control pests.

It is advisable to alternate several products to maximise their effectiveness. Avoid products also containing insecticides unless there is also a pest problem that needs control.

Plant and Fish Oil Blends (Vitax Organic 2 in 1) may also be used and are permitted under some organic regimes.

Thanks for reading 

NPK Technology

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